Method and apparatus for confirming validity of candidate cooperative device list for client cooperation in wireless communication system

ABSTRACT

Provided are a method and an apparatus for confirming the validity of a candidate cooperative device list for client cooperation in a wireless communication system. A base station searches for at least one candidate cooperation device for a source device, then an effective period timer, for an effective time command list for a list of candidate cooperation devices for a source device, is initiated, and a determination is made as to whether the list of candidate cooperation devices is valid. The base station and the source device are connected through a first system, and the base station and one recommended candidate cooperation device selected from the list of candidate cooperation devices are connected through a second system.

This application is a 35 USC §371 National Stage entry of InternationalApplication No. PCT/KR2012/004118 filed on May 24, 2012, and claimspriority to U.S. Provisional Application Nos. 61/490,073, filed May 26,2011, 61/490,075 filed May 26, 2011; 61/490,076, filed May 26, 2011, and61/494,898 filed Jun. 9, 2011, all of which are incorporated byreference in their entirety herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, a method and apparatus for confirming validity of acandidate cooperative device list for client cooperation in a wirelesscommunication system.

2. Related Art

In next generation multimedia mobile communication systems, which havebeen actively studied in recent years, there is a demand for a systemcapable of processing and transmitting a variety of information (e.g.,video and radio data) in addition to the early-stage voice service.Further, a 3rd generation wireless communication system is followed by a4th generation wireless communication which is currently being developedaiming at supporting a high-speed data service of 1 gigabits per second(Gbps) in downlink and 500 megabits per second (Mbps) in uplink. Thewireless communication system is designed for the purpose of providingreliable communication to a plurality of users irrespective of theirlocations and mobility. However, a wireless channel has an abnormalcharacteristic such as a fading phenomenon caused by a path loss, noise,and multipath, an inter-symbol interference (ISI), a Doppler effectcaused by mobility of a user equipment, etc. Therefore, varioustechniques have been developed to overcome the abnormal characteristicof the wireless channel and to increase reliability of wirelesscommunication.

Meanwhile, with the advent of a ubiquitous environment, there is a rapidincrease in a demand for receiving a seamless service anytime anywhereby using equipments. In order to satisfy such a demand, a clientcooperation technique may be introduced in a wireless communicationsystem. The client cooperation technique refers to a technique by whicha specific device helps transmission of another device. That is, onedevice may directly communicate with a base station (BS) or mayindirectly communication with the BS by the aid of another device. Theclient cooperation technique has an effect of lower power consumption,throughput enhancement, etc.

The client cooperation technique can be more effectively used in amulti-radio access technology (RAT) device. The multi-RAT device refersto a device that can operate in a plurality of communication systems.For example, the multi-RAT device can operate both in institute ofelectrical and electronics engineers (IEEE) 802.16m and IEEE 802.11. Toprovide an easiness access to the BS anytime anywhere and to maintaineffective performance, the multi-RAT device can use a multi-RAT clientcooperation technique (i.e., improved tethering) in a heterogeneousnetwork.

A base station may search for a device capable of performing clientcooperation to perform a client cooperation technology, and a sourcedevice, being connected to a searched device, may communicate with thebase station through client cooperation. A searched device may becomethe device valid or invalid for client cooperation depending on variousconditions such as movement of the source device. Therefore, it isrequired to confirm validity of discovered devices for clientcooperation and to define a time period for which information of thecorresponding devices is retained.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for confirmingvalidity of a candidate cooperative device list for client cooperationin a wireless communication system. The present invention provides amethod for confirming validity of a list of candidate cooperativedevices which can function as cooperative devices for clientcooperation. Also, a timer which specifies a time period for which acandidate cooperative device list and information on candidatecooperative devices are retained is newly defined.

In an aspect, a method for confirming, by a base station, validity of alist of candidate cooperative devices for client cooperation in awireless communication system is provided. The method includes searchingfor at least one candidate cooperative device for a source device,starting a list valid period timer indicating valid time of a candidatecooperative device list for the source device, and determining whetherthe candidate cooperative device list is valid. The base station and thesource device are connected to each other through a first system, andthe base station and one recommended candidate cooperation deviceselected from the candidate cooperation device list are connected toeach other through a second system.

The valid time may indicate a time period during which a result of thesearch for the at least one candidate cooperative device for the sourcedevice is valid.

The valid time may indicate a time period during which a neighborcandidate cooperative device list of the source device received from thesource device is valid.

The valid time may be the same or different from each other for eachcandidate cooperative device among the candidate cooperative devicelist.

If the valid time is the same for each of candidate cooperative devicesamong the candidate cooperative device list, the valid time may be oneof a minimum value, an average value, or a maximum value of valid timeof each candidate cooperative device among the candidate cooperativedevice list.

If the at least one candidate cooperative device for the source deviceis discovered while the list valid period timer is operating, the methodmay further include restarting the list valid period timer.

Whether the candidate cooperative device list is valid may be determinedwhen the list valid period timer expires.

Whether the candidate cooperative device list is valid may be determinedwhen an estimation of a location of the source device exceeds apredetermined range, or moving speed of the source device is changed.

Whether the candidate cooperative device list is valid may be determinedwhen it is determined that the candidate cooperative device list isoutdated based on a timestamp value of the candidate cooperative devicelist.

The method may further include starting a list holding timer within thebase station, and holding information on the candidate cooperativedevice list and the source device for a time period indicated by thelist holding timer.

Time indicated by the list holding timer may be defined by a pre-fixedvalue.

The method may further include transmitting the time period indicated bythe list holding timer to the source device.

The list holding timer may start when the candidate cooperative devicelist is updated.

If the list holding timer expires, the method may further includedeleting the information on the candidate cooperative device list andthe source device.

The first system may be one of institute of electrical and electronicsengineers (IEEE) 802.16, IEEE 802.16m, IEEE 802.20, evolved-UMTSterrestrial radio access (E-UTRA), 3^(rd) generation partnership project(3GPP) long-term evolution (LTE), or 3GPP LTE-A (advanced), and thesecond system may be IEEE 802.11.

In another aspect, a base station for client cooperation in a wirelesscommunication system is provided. The base station includes a radiofrequency (RF) unit for transmitting or receiving a radio signal, and aprocessor connected to the RF unit, and configured to search for atleast one candidate cooperative device for a source device, start a listvalid period timer indicating valid time of a candidate cooperativedevice list for the source device, and determine whether the candidatecooperative device list is valid. The base station and the source deviceare connected to each other through a first system, and the base stationand one recommended candidate cooperation device selected from thecandidate cooperation device list are connected to each other through asecond system.

A list for candidate cooperative devices for client cooperation can bemanaged in an effective manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows an example of a frame structure of IEEE 802.16m.

FIG. 3 shows an example of a frame structure of IEEE 802.11.

FIG. 4 shows an example of implementing a client cooperation technique.

FIG. 5 shows another example of implementing a client cooperationtechnique.

FIG. 6 shows another example of implementing a client cooperationtechnique.

FIG. 7 shows an example of a method for setting a connection accordingto an embodiment of the present invention.

FIG. 8 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

FIG. 9 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

FIG. 10 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

FIG. 11 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

FIG. 12 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A technology below can be used in a variety of wireless communicationsystems, such as code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), and singlecarrier frequency division multiple access (SC-FDMA). CDMA can beimplemented using radio technology, such as universal terrestrial radioaccess (UTRA) or CDMA2000. TDMA can be implemented using radiotechnology, such as global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). OFDMA can be implemented using radio technology, suchas IEEE 802.11(Wi-Fi), IEEE 802.16(WiMAX), IEEE 802-20, or Evolved UTRA(E-UTRA). IEEE 802.16m is the evolution of IEEE 802.16e, and it providesa backward compatibility with an IEEE 802.16e-based system. UTRA is partof a universal mobile telecommunications system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is part of evolvedUMTS (E-UMTS) using evolved-UMTS terrestrial radio access (E-UTRA), andit adopts OFDMA in downlink (DL) and SC-FDMA in uplink (UL). LTE-A(advanced) is the evolution of 3GPP LTE.

IEEE 802.16m and IEEE 802.11 are chiefly described as an example inorder to clarify the description, but the technical spirit of thepresent invention is not limited to IEEE 802.16m and IEEE 802.11.

FIG. 1 shows a wireless communication system.

Referring to FIG. 1, the wireless communication system 10 includes oneor more base stations (BSs) 11. The BSs 11 provide communicationservices to respective geographical areas (in general called ‘cells’) 15a, 15 b, and 15 c. Each of the cells can be divided into a number ofareas (called ‘sectors’). A user equipment (UE) 12 can be fixed ormobile and may be referred to as another terminology, such as a mobilestation (MS), a mobile terminal (MT), a user terminal (UT), a subscriberstation (SS), a wireless device, a personal digital assistant (PDA), awireless modem, or a handheld device. In general, the BS 11 refers to afixed station that communicates with the UEs 12, and it may be referredto as another terminology, such as an evolved-NodeB (eNB), a basetransceiver system (BTS), or an access point.

The UE generally belongs to one cell. A cell to which a UE belongs iscalled a serving cell. A BS providing the serving cell withcommunication services is called a serving BS. A wireless communicationsystem is a cellular system, and so it includes other cells neighboringa serving cell. Other cells neighboring the serving cell are calledneighbor cells. A BS providing the neighbor cells with communicationservices is called as a neighbor BS. The serving cell and the neighborcells are relatively determined on the basis of a UE.

This technology can be used in the downlink (DL) or the uplink (UL). Ingeneral, DL refers to communication from the BS 11 to the UE 12, and ULrefers to communication from the UE 12 to the BS 11. In the DL, atransmitter may be part of the BS 11 and a receiver may be part of theUE 12. In the UL, a transmitter may be part of the UE 12 and a receivermay be part of the BS 11.

FIG. 2 shows an example of a frame structure of IEEE 802.16m.

Referring to FIG. 2, a superframe (SF) includes a superframe header(SFH) and four frames F0, F1, F2, and F3. Each frame may have the samelength in the SF. Although it is shown that each SF has a length of 20milliseconds (ms) and each frame has a length of 5 ms, the presentinvention is not limited thereto. A length of the SF, the number offrames included in the SF, the number of SFs included in the frame, orthe like can change variously. The number of SFs included in the framemay change variously according to a channel bandwidth and a cyclicprefix (CP) length.

One frame includes 8 subframes SF0, SF1, SF2, SF3, SF4, SFS, SF6, andSF7. Each subframe can be used for uplink or downlink transmission. Onesubframe includes a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain, and includes a pluralityof subcarriers in a frequency domain. An OFDM symbol is for representingone symbol period, and can be referred to as other terminologies such asan OFDM symbol, an SC-FDMA symbol, etc., according to a multiple accessscheme. The subframe can consist of 5, 6, 7, or 9 OFDMA symbols.However, this is for exemplary purposes only, and thus the number ofOFDMA symbols included in the subframe is not limited thereto. Thenumber of OFDMA symbols included in the subframe may change variouslyaccording to a channel bandwidth and a CP length. A subframe type may bedefined according to the number of OFDMA symbols included in thesubframe. For example, it can be defined such that a type-1 subframeincludes 6 OFDMA symbols, a type-2 subframe includes 7 OFDMA symbols, atype-3 subframe includes 5 OFDMA symbols, and a type-4 subframe includes9 OFDMA symbols. One frame may include subframes each having the sametype. Alternatively, one frame may include subframes each having adifferent type. That is, the number of OFDMA symbols included in eachsubframe may be identical or different in one frame. Alternatively, thenumber of OFDMA symbols included in at least one subframe of one framemay be different from the number of OFDMA symbols of the remainingsubframes of the frame.

Time division duplex (TDD) or frequency division duplex (FDD) may beapplied to the frame. In the TDD, each subframe is used in uplink ordownlink transmission at the same frequency and at a different time.That is, subframes included in a TDD frame are divided into an uplinksubframe and a downlink subframe in the time domain. A switching pointrefers to a point where a transmission direction is changed from anuplink region to a downlink region or from a downlink region to anuplink region. In the TDD, the number of the switching points in eachframe may be two. In the FDD, each subframe is used in uplink ordownlink transmission at the same time and at a different frequency.That is, subframes included in an FDD frame are divided into an uplinksubframe and a downlink subframe in the frequency domain. Uplinktransmission and downlink transmission occupy different frequency bandsand can be simultaneously performed.

One OFDMA symbol includes a plurality of subcarriers. The number ofsubcarriers is determined by a fast Fourier transform (FFT) size. Thesubcarrier can be classified into a data subcarrier for datatransmission, a pilot subcarrier for various estimations, and a nullsubcarrier for a guard band and a direct current (DC) carrier. The OFDMAsymbol is characterized by parameters BW, N_(used), n, G, etc. Theparameter BW denotes a nominal channel bandwidth. The parameter N_(used)denotes the number of used subcarriers (including the DC subcarrier).The parameter n denotes a sampling factor. The parameter n is combinedwith the parameters BW and N_(used) to determine a subcarrier spacingand a useful symbol time. The parameter G denotes a ratio of a cyclicprefix (CP) time and a useful time.

Table 1 below shows an orthogonal frequency division multiple access(OFDMA) parameter.

TABLE 1 Channel bandwidth, BW(MHz) 5 7 8.75 10 20 Sampling factor, n28/25 8/7 8/7 28/25 28/25 Sampling frequency, F_(s)(MHz) 5.6 8 10 11.222.4 FFT size, N_(FFT) 512 1024 1024 1024 2048 Subcarrier spacing,Δf(kHz) 10.94 7.81 9.77 10.94 10.94 Useful symbol time, T_(b)(μs) 91.4128 102.4 91.4 91.4 G = ⅛ Symbol time, T_(s)(μs) 102.857 144 115.2102.857 102.857 FDD Number of 48 34 43 48 48 ODFMA symbols per 5 msframe Idle time(μs) 62.857 104 46.40 62.857 62.857 TDD Number of 47 3342 47 47 ODFMA symbols per 5 ms frame TTG + RTG(μs) 165.714 248 161.6165.714 165.714 G = 1/16 Symbol time, T_(s)(μs) 97.143 136 108.8 97.14397.143 FDD Number of 51 36 45 51 51 ODFMA symbols per 5 ms frame Idletime(μs) 45.71 104 104 45.71 45.71 TDD Number of 50 35 44 50 50 ODFMAsymbols per 5 ms frame TTG + RTG(μs) 142.853 240 212.8 142.853 142.853 G= ¼ Symbol time, T_(s)(μs) 114.286 160 128 114.286 114.286 FDD Number of43 31 39 43 43 ODFMA symbols per 5 ms frame Idle time(μs) 85.694 40 885.694 85.694 TDD Number of 42 30 38 42 42 ODFMA symbols per 5 ms frameTTG + RTG(μs) 199.98 200 136 199.98 199.98 Number of Guard Left 40 80 8080 160 subcarriers Right 39 79 79 79 159 Number of used subcarriers 433865 865 865 1729 Number of PRU in type-1 subframe 24 48 48 48 96 InTable 1, N_(FFT) denotes a smallest power of 2 greater than N_(used). Asampling factor is defined as F_(s) = floor(n · BW/8000) × 8000. Asubcarrier spacing is defined as Δf = F_(s)/NFFT. A useful symbol timeis defined as T_(b) = 1/Δf. A CP time is defined as T_(g) = G · T_(b).An OFDMA symbol time is defined as T_(s) = T_(b) + T_(g). A samplingtime is defined as T_(b)/N_(FFT).

FIG. 3 shows an example of a frame structure of IEEE 802.11.

A frame of IEEE 802.11 includes a set of fields in a fixed order.Referring to FIG. 3, the frame of IEEE 802.11 includes a frame controlfield, a duration/ID field, an address 1 field, an address 2 field, anaddress 3 field, a sequence control field, an address 4 field, a qualityof service (QoS) control field, an HT control field, a frame body field,and a frame check sequence (FCS) field. Among the fields listed above,the frame control field, the duration/ID field, the address 1 field, andthe FCS field constitute a minimum IEEE 802.11 frame format, and may beincluded in all IEEE 802.11 frames. The address 2 field, the address 3field, the sequence control field, the address 4 field, the QoS controlfield, the HT control field, and the frame body field may be includedonly in a specific frame type.

The frame control field may include various subfields. The duration/IDfield may be 16 bits in length. The address field may include a basicservice set identifier (BSSID), a source address (SA), a destinationaddress (DA), a transmitting STA address (TA), and a receiving STAaddress (RA). In the address field, different fields may be used forother purposes according to a frame type. The sequence control field canbe used when fragments are reassembled or when an overlapping frame isdiscarded. The sequence control field may be 16 bits, and may includetwo subfields indicating a sequence number and a fragment number. TheFCS field can be used to check an error of a frame received by astation. The FCS field may be a 32-bit field including a 32-bit cyclicredundancy check (CRC). An FCS can be calculated across the frame bodyfield and all fields of a media access control (MAC) header.

The frame body field may include information specified for an individualframe type and subtype. That is, the frame body field carries high-leveldata from one station to another station. The frame body field can alsobe called a data field. The frame body field can be variously changed inlength. A minimum length of the frame body field may be zero octet. Amaximum length of the frame body field may be determined by a total sumof a maximum length of a MAC service data unit (MSDU), a length of amesh control field, and an overhead for encryption or a total sum of amaximum length of an aggregated MSDU (A-MSDU) and an overhead forencryption. The data frame includes high-level protocol data of theframe body field. The data frame may always include the frame controlfield, the duration/ID field, the address 1 field, the address 2 field,the address 3 field, the sequence control field, the frame body field,and the FCS field. A presence of an address 4 field may be determined bya configuration of a ‘To DS’ subfield and a ‘From DS’ subfield in theframe control field. Another data frame type can be categorizedaccording to a function.

A management frame may always include the frame control field, theduration/ID field, the address 1 field, the address 2 field, the address3 field, the sequence control field, the frame body field, and the FCSfield. Data included in the frame body field generally uses afixed-length field called a fixed field and a variable-length fieldcalled an information element. The information element is avariable-length data unit.

The management frame can be used for various purposes according to asubtype. That is, a frame body field of a different subtype includesdifferent information. A beacon frame reports an existence of a network,and takes an important role of network maintenance. The beacon framecorresponds to a parameter which allows a mobile station to participatein the network. In addition, the beacon frame is periodicallytransmitted so that the mobile station can scan and recognize thenetwork. A probe request frame is used to scan an IEEE 802.11 network inwhich the mobile station exists. A probe response frame is a responsefor the probe request frame. An authentication request is used so thatthe mobile station requests an access point to perform authentication.An authentication response frame is a response for the authenticationrequest frame. A deauthentication frame is used to finish anauthentication relation. An association request frame is transmitted sothat the mobile station participates in the network when the mobilestation recognizes the compatible network and is authenticated. Anassociation response frame is a response for the association requestframe. A deassociation frame is used to finish an association relation.

Three states may exist according to an authentication and associationprocedure in IEEE 802.11. Table 2 below shows the three states of IEEE802.11.

TABLE 2 Authentication Association State 1 X X State 2 ◯ X State 3 ◯ ◯

To transmit a data frame, a device must perform the authentication andassociation procedure with respect to a network. In Table 2, a procedureof transitioning from the state 1 to the state 2 can be called theauthentication procedure. The authentication procedure can be performedin such a manner that one device acquires information of a differentdevice and authenticates the different device. The information of thedifferent device can be acquired by using two methods, i.e., a passivescanning method for acquiring information of a different node byreceiving a beacon frame and an active scanning method for acquiring theinformation of the different device by transmitting a probe requestmessage and receiving a probe response message received in responsethereto. The authentication procedure can be complete by exchanging anauthentication request frame and an authentication response frame.

In Table 2, a procedure of transitioning from the state 2 to the state 3can be called the association procedure. The association procedure canbe complete when two devices exchange the association request frame andthe association response frame upon completion of the authenticationprocedure. An association ID can be allocated by the associationprocedure.

A client cooperation technique may be introduced in a wirelesscommunication system. One device may directly communicate with a basestation (BS) or may indirectly communication with the BS by the aid ofanother device. Hereinafter, a source device refers to a device whichcommunicates with the BS through a connection with another device. Acooperative device refers to a relay entity which helps the sourcedevice to communicate with the BS. The client cooperation technique hasan effect of lower power consumption. In terms of a device, a path-losscan be decreased by the client cooperation technique, thereby being ableto decrease transmit power. In terms of a network, total network powerconsumption can be decreased. In addition, the client cooperationtechnique has an effect of throughput enhancement. In terms of a device,a source device can use a good-quality link between a cooperative deviceand a BS and between BSs. In addition, an antenna extension gain can beobtained. In terms of the network, network capacity can be increased byusing client clustering based on frequency reuse without an additionalinfrastructure.

FIG. 4 shows an example of implementing a client cooperation technique.

Referring to FIG. 4, in the client cooperation technique, a sourcedevice can directly communicate with a macro BS, or can communicate withthe macro BS via a cooperative device. The cooperative device maydirectly communicate with the macro BS, or can help communication of thesource device. This is different from a mobile relay in a sense that thesource device can directly communicate with the macro BS. In this case,each device and the macro BS can communicate by using a first radioaccess technology (RAT), and the source device and the cooperativedevice can communicate by using a second RAT. The first RAT may be aradio technology such as IEEE 802.16 (WiMAX), IEEE 802.16m or IEEE802.20, etc.

Alternatively, the first RAT may be a radio technology such as E-UTRA,3GPP LTE or 3GPP LTE-A, etc. The second RAT may be IEEE 802.11.

FIG. 5 shows another example of implementing a client cooperationtechnique.

The client cooperation technique can be more effectively used in amulti-RAT device. The multi-RAT device refers to a device that canoperate in a plurality of communication systems. For example, themulti-RAT device can operate both in IEEE 802.16m and IEEE 802.11. Whenthe multi-RAT device uses the client cooperation technique, themulti-RAT device can communicate with an IEEE 802.16m BS by using aplurality of RATs. For example, as shown in FIG. 5, if channel qualityis poor between a second device and a BS or if the second device locatedin a shadow area cannot receive a signal from the BS, the first devicecan be used as a cooperative device to communicate with the BS. In thiscase, each device and the BS can communicate by using the first RAT, andthe source device and the cooperative device can communicate by usingthe second RAT. The first RAT may be a radio technique such as IEEE802.16, IEEE 802.16m, IEEE 802.20, E-UTRA, 3GPP LTE or 3GPP LTE-A, etc.The second RAT may be IEEE 802.11.

FIG. 6 shows another example of implementing a client cooperationtechnique.

A source device and a macro BS (e.g., IEEE 802.16m BS) can be connectedthrough a direct link, and can be connected through an indirect link byusing a cooperative device. In this case, each device and the BS can beconnected by using IEEE 802.16m, and the source device and thecooperative device can be connected by using IEEE 802.11.

In what follows, a method for connecting multi-RAT devices to each otherto perform a client cooperation technology will be described.Hereinafter, it is assumed that a multi-RAT device attempting to connectto a first RAT base station through client cooperation is denoted as asource device while a device that may be connected to the source devicethrough a second RAT for client cooperation is denoted as a candidatecooperative device. When a candidate cooperative device is connected toa source device through the second RAT and performs client cooperation,the candidate cooperative device may be a cooperative device of clientcooperation. In what follows, it is assumed that the first RAT is theIEEE 802.16m, and the second RAT is the IEEE 802.11, but the presentinvention is not limited to thereto.

Before details of a connection procedure for multi-RAT devices, a devicediscovery procedure is described first. A device discovery procedure maybe carried out before a connection procedure. Through the devicediscovery procedure, a base station and/or a source device may searchfor a candidate cooperative device which may function as a cooperativedevice for client cooperation. In general, the device discoveryprocedure may be performed by exchanging a beacon message or a proberequest/response message.

The base station searches for a candidate cooperative device and informsthe source device of a list including one or more candidate cooperativedevices. The list may include information such as a device identifier(ID), a media access control (MAC) address, a beacon interval, and soon. Also, the base station may inform the source device of a candidatecooperative device deemed relatively suitable for performing clientcooperation with the source device. Such a kind of device may be calleda recommended candidate cooperative device. In other words, recommendedcooperative devices may form a subset of a list including at leastcandidate cooperative devices. A recommended device may be determinedbased on a total number of source devices, speed of source devices, andso on.

A source device may utilize information about candidate cooperativedevices or recommended cooperative devices provided by a base station.The source device may transmit a unicast probe request message to eachof candidate cooperative devices or recommended cooperative devices andmay perform a subsequent procedure. The source device may ignore abeacon message transmitted from a different device other than each ofcandidate cooperative devices or recommended cooperative devices.

In what follows, a connection procedure between a source device and acandidate cooperative device will be described. The connection proceduremay be initiated by a source device or a base station. If the connectionprocedure is initiated by a source device, the source device may requesta base station to start the connection procedure and set up a connectionto a candidate cooperative device which can support client cooperation.If the connection procedure is initiated by a base station, the basestation may request a source device to start the connection procedure,and the source device may set up a connection to a candidate cooperativedevice which can support client cooperation. In general, the devicediscovery procedure may be carried out by exchanging an authenticationrequest/response message or an association request/response message.

1) First of all, a case where the connection procedure is initiated by asource device is described.

FIG. 7 shows an example of a method for setting a connection accordingto an embodiment of the present invention.

In step S100, at least one candidate cooperative device is discovered asa result of a device discovery procedure. Once the device discoveryprocedure is completed, a list holding timer within the base station andthe source device starts. While the list holding timer is operating, thelist of candidate cooperative devices within each of the base stationand the source device is retained.

In step S110, the source device transmits a first activation requestmessage to the base station. The first activation request message mayinclude the following parameters.

-   -   A list of candidate cooperative devices maintained by the source        device: this list may be included in the first activation        request message only when the list holding timer expires, or the        list of candidate cooperative devices is changed. The list of        candidate cooperative devices maintained by the source device        may be changed when the source device reconfirms the candidate        cooperative devices independently.    -   A list of neighbor devices maintained by the source device: this        list may be a list of neighbor devices discovered by the source        device without assistance of the base station. In other words,        the list may be a list of devices not identified as a candidate        cooperative device.    -   The latest timestamp of a list of candidate cooperative devices        maintained by the source device: this parameter indicates one        particular time point at which the list of candidate cooperative        devices is updated.    -   Information on location of the source device: this information        may be included if the source device's location has been        changed. The information on location of the source device may        include at least one of absolute location of the source device        or relative location of a neighbor IEEE 802.11 access point        (AP).    -   Indication of whether to maintain an IEEE 802.16m connection:        this parameter indicates whether to maintain a connection        between the source device and the base station if a connection        between the source device and the candidate cooperative devices        is established, and thus client cooperation is carried out.

Referring to FIG. 7 again, in step S120, upon receiving the firstactivation request message from the source device, the base stationdetermines whether the list of candidate cooperative devices is valid.If it is determined that the list of candidate cooperative devices isvalid, the base station may select candidate cooperative devicesappropriate for client cooperation. At this time, the base station mayselect candidate cooperative devices appropriate for client cooperationbased on the list of candidate cooperative devices received from thesource device (if the list is included in the first activation requestmessage), information on at least one candidate cooperative devicewithin the list of candidate cooperative devices maintained by the basestation (if the list holding timer is in operation), or information onlocation of the source device, etc. Also, the base station may selectcandidate cooperative devices appropriate for client cooperation basedon a total number of source devices, speed of source devices, etc.

In step S130, the base station transmits a second activation requestmessage to the selected candidate cooperative device. In step S140, thebase station receives from the selected candidate cooperative device anactivation response message as a response to the second activationrequest message. By exchanging the second activation request message andthe activation response message, the base station may negotiate theselected candidate cooperative device about an activation time point ofradio frequency (RF) for multi-RAT client cooperation, capability ofperforming client cooperation, etc.

In step S150, the base station transmits an activation command messageto the source device. The base station may inform the source device ofdecisions made between the base station and the candidate cooperativedevice through the activation command message. In other words, theactivation command message may include a result of the activationrequest of the source device. If the result of the activation request ofthe source device is successful, the activation command message mayinclude the following parameters.

-   -   Multi-RAT information of a candidate cooperative device capable        of performing client cooperation: this information may include        the MAC address of the candidate cooperative device, type and        version of a system type which allows operation of the candidate        cooperative device, security association, etc.    -   Multi-RAT random access time: for example, this may be a frame        offset or a frame number.    -   Indication of whether to maintain IEEE 802.16m connection: this        parameter indicates whether to continuously maintain a        connection between the source device and the base station if a        connection between the source device and the candidate        cooperative device is completed and thus client cooperation is        performed. If the IEEE 802.16m connection between the source        device and the base station is not maintained, the activation        command message may include action time which indicates absolute        time or relative time, or both at which a data and a control        signal are transmitted through a multi-RAT connection, and        disconnection time which indicates absolute time or relative        time, or both at which the IEEE 802.16m connection is        terminated.

Upon receiving the activation command message, in step S160, the sourcedevice may perform an authentication and an association procedure forthe candidate cooperative device.

In step S170, the source device and the candidate cooperative device mayeach transmit a connection complete request message for clientcooperation to the base station. Through the connection complete requestmessage, a result of the performing the authentication and theassociation procedure may be transmitted. Also, the connection completerequest message transmitted by the source device may include theparameter indicating whether to maintain the IEEE 802.16m connection.

In step S180, the base station transmits a connection complete responsemessage for client cooperation to each of the source device and thecandidate cooperative device. The connection complete response messagemay include an address to be used for multi-RAT client cooperation,security information of the IEEE 802.16m, etc. The address maycorrespond to a local ID dedicated to client cooperation or a local IDof the source device. The connection compete response message mayinclude a parameter indicating whether to maintain the IEEE 802.16mconnection.

Meanwhile, the base station may receive the connection complete requestmessage from one or more candidate cooperative devices. Then the basestation may select one candidate cooperative device and transmit theconnection complete response message thereto. Also, in case ofdevice-to-device direct communication, the corresponding device and basestation may select a plurality of devices and set up a connection witheach of the devices.

FIG. 8 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

In step S200, at least one candidate cooperative device is discoveredfrom the device discovery procedure. In step S210, the source devicetransmits a first activation request message to the base station. Asdescribed above in FIG. 7, the first activation request message mayinclude a list of candidate cooperative devices maintained by the sourcedevice, a list of neighbor devices maintained by the source device, thelatest timestamp of the list of candidate cooperative devices maintainedby the source device, information on location of the source device,indication of whether to maintain IEEE 802.16m connection, etc. In stepS220, upon receiving the first activation request message from thesource device, the base station determines whether the list of candidatecooperative devices is valid.

The base station may determine that the list of candidate cooperativedevices is not valid. Then, in step S230, the base station may checkexistence of other devices appropriate for client cooperation. Forexample, if the base station receives a list of neighbor devicesmaintained by the source device through the first activation requestmessage, the base station may determine whether a device belonging tothe corresponding list is appropriate as a cooperative device. Accordingto the checking result, the base station may select a candidatecooperative device appropriate for client cooperation. Similarly, if thebase station receives information on location of the source devicethrough the first activation request message, the base station mayselect a candidate cooperative device for client cooperation based onthe absolute location or relative location of the source device, orboth.

In step S240, the base station transmits a second activation requestmessage to the selected candidate cooperative device. In step S250, thebase station receives an activation response message as a response tothe second activation request message from the selected candidatecooperative device. By exchanging the second activation request messageand the activation response message, the base station may negotiate theselected candidate cooperative device about activation time of RF formulti-RAT client cooperation, capability of performing clientcooperation, etc. In step S260, the base station transmits an activationcommand message to the source device. The activation command message mayinclude a result of the activation request of the source device. If theresult of the activation request of the source device is successful, theactivation command message may include parameters such as multi-RATinformation of the candidate cooperative device capable of performingclient cooperation, multi-RAT random access time, an indicator ofwhether to maintain the IEEE 802.16m connection, etc., as describedabove in FIG. 7.

Upon receiving an activation command message, in step S270, the sourcedevice may perform an authentication and an association procedure forthe candidate cooperative device. In step S280, the source device andthe candidate cooperative device may each transmit a connection completerequest message for client cooperation to the base station. Through theconnection complete request message, a result of performing theauthentication and the association procedure may be transmitted. In stepS290, the base station a connection complete response message for clientcooperation to each of the source device and the candidate cooperativedevice. The connection complete request message may include and addressused for multi-RAT client cooperation, security information of the IEEE802.16m, a parameter indicating whether to maintain the IEEE 802.16mconnection, etc.

FIG. 9 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

In step S300, as a result of the device discovery procedure, at leastone candidate cooperative device is discovered. In step S310, the sourcedevice transmits the first activation request message to the basestation. As described above in FIG. 7, the first activation requestmessage may include such parameters as a list of candidate cooperativedevices maintained by the source device, a list of neighbor devicemaintained by the source device, the latest timestamp of the list ofcandidate cooperative devices maintained by the source device,information about location of the source device, an indicator of whetherto maintain the IEEE 802.16m connection, etc. In step S320, uponreceiving the first activation request message from the source device,the base station determines whether the list of candidate cooperativedevices is valid.

The base station may determine that the list of candidate cooperativedevices is not valid. Also, the base station may not be able to performa connection procedure since no device appropriate for clientcooperation is available. In this case, in step S330, the base stationtransmits an activation command message to the source device. Theactivation command message may indicate a failure of the activationrequest from the source device. Also, the activation command message mayinclude a cause of the failure (e.g., no valid list or no newly updatedlist) and an action code (re-execution of the device discoveryprocedure). In step S340, upon receiving the activation command message,the source device may perform the device discovery procedure again.

1) In what follows, the case where the connection procedure is initiatedby the base station is described. The base station may communicate thesource device by performing client cooperation by taking account of lowlink quality between the base station and the source device and othersimilar reasons. The method for setting a connection described above inFIGS. 7 to 9 may be applied similarly to the case where the connectionprocedure is initiated by the base station.

FIG. 10 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

Once the list of candidate cooperative devices is validated, the basestation may select candidate cooperative devices appropriate for clientcooperation. At this time, the base station may select candidatecooperative devices appropriate for client cooperation based on speed,timestamp, and location information, etc., of the source device.

In step S400, the base station transmits an activation request messageto the selected candidate cooperative device. In step S410, the basestation receives from the selected candidate cooperative device anactivation response message as a response to the activation requestmessage. By exchanging the activation request message and the activationresponse message, the base station may negotiate the candidatecooperative device about an activation time point of RF for multi-RATclient cooperation, capability of performing client cooperation, etc. Instep S420, the base station transmits an activation command message tothe source device. The activation command message may include a resultof the activation request of the source device. If the result of theactivation request of the source device is successful, the activationcommand message may include such parameters as multi-RAT information ofa candidate cooperative device capable of performing client cooperation,multi-RAT arbitrary connection time, an indicator of whether to maintainIEEE 802.16m connection, etc.

Upon receiving the activation command message, in step S430, the sourcedevice may perform an authentication and an association procedure forthe candidate cooperative device. In step S440, the source device andthe candidate cooperative device may each transmit a connection completerequest message for client cooperation to the base station. Through theconnection complete request message, a result of performing theauthentication and the association procedure may be transmitted. In stepS450, the base station transmits a connection complete response messagefor client cooperation to each of the source device and the candidatecooperative device. The connection complete response message may includean address to be used for multi-RAT client cooperation, securityinformation of the IEEE 802.16m, an indicator of whether to maintain theIEEE 802.16m connection, etc.

FIG. 11 shows an example of a method for setting a connection accordingto another embodiment of the present invention.

The base station may determine that the list of candidate cooperativedevices is not valid. In step S500, the base station transmits a firstactivation request message to the source device.

Upon receiving the first activation request message, in step S510, thesource device performs the device discovery procedure. In step S520, thesource device transmits the first activation response message to thebase station as a response to the first activation request message. Atthis time, the first activation response message may include suchparameters as a list of candidate cooperative devices maintained by thesource device, a list of neighbor devices maintained by the sourcedevice, the latest timestamp of the list of candidate cooperativedevices maintained by the source device, information on location of thesource device, an indicator of whether to maintain the IEEE 802.16m,etc.

Similarly, the source device may transmit the first activation responsemessage including information for generating a list of candidatecooperative devices directly to the base station without performing thedevice discovery procedure. The information for generating the list ofcandidate cooperative devices may include location information of thesource device.

Upon receiving the first activation response message from the sourcedevice, the base station may select a candidate cooperative deviceappropriate for client cooperation. In step S530, the base stationtransmits a second activation request message to the selected candidatecooperative device. In step S540, the base station receives a secondactivation response message as a response to the second activationrequest message from the selected candidate cooperative device. Byexchanging the second activation request message and the secondactivation response message, the base station may negotiate consult thecandidate cooperative device about an activation time point of RF formulti-RAT client cooperation, capability of performing clientcooperation, etc. In step S550, the base station transmits an activationcommand message to the source device. The activation command message mayinclude a result of the activation request of the source device. If theresult of the activation request of the source device is successful, theactivation command message may include such parameters as multi-RATinformation of a candidate cooperative device capable of performingclient cooperation, multi-RAT arbitrary connection time, an indicator ofwhether to maintain the IEEE 802.16m, etc.

Upon receiving an activation command message, in step S560, the sourcedevice may perform an authentication and an association procedure forthe candidate cooperative device. In step S570, the source device andthe candidate cooperative device may each transmit a connection completerequest message for client cooperation to the base station. Through theconnection complete request message, a result of performing theauthentication and the association procedure may be transmitted. In stepS580, the base station a connection complete response message for clientcooperation to each of the source device and the candidate cooperativedevice. The connection complete request message may include and addressused for multi-RAT client cooperation, security information of the IEEE802.16m, a parameter indicating whether to maintain the IEEE 802.16mconnection, etc.

In what follows, a procedure for checking validity of a list ofcandidate cooperative devices and holding time of each of the lists ofcandidate cooperative devices described in step S120 of FIG. 7, etc, isdescribed in more detail.

The list of candidate cooperative devices may not be valid any time goeson, due to the effect of moving speed and/or location of the sourcedevice and each of candidate cooperative devices. For example, eventhough candidate cooperative devices which can support clientcooperation of the source device have been discovered, the list ofdiscovered candidate cooperative devices may not be valid any more asthe source device moves. Therefore, it is necessary to check validity ofthe list of candidate cooperative devices before the source device andthe candidate cooperative devices are connected to each other.

Also, the base station stores within itself a search result for eachcandidate cooperative device obtained through the device discoveryprocedure, namely a list of candidate cooperative devices for a singlesource device and information on APs adjacent to the source device.However, storing all of the device search results within the basestation is inefficient in terms of storage space and costs. In addition,it is not necessary any more to store information on candidatecooperative devices of a source device for which direct connection tothe base station is more advantageous and information on candidatecooperative devices of a source device which has no further data tocommunicate. Therefore, the base station needs to determine until whenthe search result for each of candidate cooperative devices has to bestored, and the source device needs to know until when its search resultwill be stored in the base station. Therefore, in addition to checkingvalidity of the list of candidate cooperative devices, it may berequired to determine holding time for the list of candidate cooperativedevices with respect to the source device.

Validity of a list of candidate cooperative devices may be indicated bya list valid period timer within the base station and the source device.In other words, the list valid period timer indicates a time periodduring which a device discovery result is valid. Valid time of a listmay be the same or different for each candidate cooperative device. Ifthe valid time for each list of candidate cooperative devices is thesame for each candidate cooperative device, the valid time may assumeone from among the minimum value, average value, and maximum value ofvalid time of multiple lists of candidate cooperative devices. If eachvalid time of the lists of candidate cooperative devices is differentfrom each other for the respective candidate cooperative devices, thevalid time may be determined based on moving speed of the source deviceand/or each candidate cooperative device transmitted from the basestation, or the valid time may be transmitted explicitly from the basestation.

If a list of candidate cooperative devices is obtained while the listvalid period timer is a deactivation state, the source device and thebase station may configure and start their list valid period timer byusing a predetermined value. Such a case may happen when the devicediscovery procedure is performed for the first time to obtain a list ofcandidate cooperative devices. If the device discovery result is updatedwhile the list valid period timer is not terminated, the source deviceand the base station may reconfigure and then restart the list validperiod timer. Similarly, if the list valid period timer expires withouta further update of the device discovery result, the source device andthe base station deactivates the list valid period timer.

Validity of a list of candidate cooperative devices may be checked bythe source device or the base station. The source device may checkvalidity of the list of candidate cooperative devices according to acondition of whether the list valid period timer is terminated, whethermoving speed of the source device changes, or a timestamp value of thelist of candidate cooperative devices maintained by the source device.If the condition above is met, the source device may perform the devicediscovery procedure again, figure out its absolute location and/orrelative location, or figure out the receive state of a beacon message.Such a procedure may be performed before the connection procedurebetween the source device and the candidate cooperative device.

Also, the base station may determine that information of thecorresponding list is outdated based on a condition of whether the listvalid period timer is terminated, or whether moving speed of the sourcedevice is changed, or according to the timestamp value of the list ofcandidate cooperative devices received through an activation requestmessage during the connection procedure between devices for clientcooperation. Similarly, the base station may check validity of the listof candidate cooperative devices according to a condition of whether anestimate of a movement radius of devices due to moving speed thereofexceeds a predetermined range. If the condition above is met, the basestation may make the source device perform the device discoveryprocedure again or may request transmission of location information ofthe source device. Similarly, the base station may command the candidatecooperative device to report whether the source device is found.

Meanwhile, separately from the list valid period timer, a list holdingtimer may be defined. A list holding timer indicates holding time duringwhich the base station stores a search result for a particular candidatecooperative device, namely a list of candidate cooperative devices andinformation of neighbor APs of a particular source device. The holdingtime may be defined by a predetermined value or may be delivered to thesource device by the base station. If the holding time is delivered tothe source device, the holding time may be determined based on movingspeed, link quality, etc., of the source device. The source device andthe base station start the list holding timer at the time when the listof candidate cooperative devices is updated and/or communicated. At thistime, if the list holding timer is already in operation, thecorresponding timer is reconfigured and starts over. Similarly, if thelist holding timer expires, the base station may delete a search resultfor the corresponding source device and information on the correspondingsource device.

If the list holding timer is still in operation and the source devicetransmits an activation request message for client cooperation to thebase station, the activation request message may not include the list ofcandidate cooperative devices maintained by the source device. This isso because the list holding timer in the source device and the basestation is still in operation, and thus the list of candidatecooperative devices is still stored within the base station. On theother hand, if the list holding timer expires and the source devicetransmits an activation request message for client cooperation to thebase station, the activation request message may include the list ofcandidate cooperative devices maintained by the source device. This isso because the base station no longer stores a device discovery resultof the corresponding source device. Meanwhile, in the case of the abovesituation, the list of candidate cooperative devices is considered to bevalid.

FIG. 12 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

ABS 800 includes a processor 810, a memory 820, and a radio frequency(RF) unit 830. The processor 810 may be configured to implement proposedfunctions, procedures, and/or methods in this description. Layers of theradio interface protocol may be implemented in the processor 810. Thememory 820 is operatively coupled with the processor 810 and stores avariety of information to operate the processor 810. The RF unit 830 isoperatively coupled with the processor 810, and transmits and/orreceives a radio signal.

A UE 900 may include a processor 910, a memory 920 and a RF unit 930.The processor 910 may be configured to implement proposed functions,procedures and/or methods described in this description. Layers of theradio interface protocol may be implemented in the processor 910. Thememory 920 is operatively coupled with the processor 910 and stores avariety of information to operate the processor 910. The RF unit 930 isoperatively coupled with the processor 910, and transmits and/orreceives a radio signal.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method for confirming, by a base station,validity of a list of candidate cooperative devices for clientcooperation in a wireless communication system, the method comprising:searching for at least one candidate cooperative device for a sourcedevice; starting a list valid period timer indicating valid time of acandidate cooperative device list for the source device; determiningwhether the candidate cooperative device list is valid; and restartingthe list valid period timer if the at least one candidate cooperativedevice for the source device is discovered while the list valid periodtimer is operating, wherein the base station and the source device areconnected to each other through a first system, and wherein the basestation and the candidate cooperation device are connected to each otherthrough a second system.
 2. The method of claim 1, wherein the validtime indicates a time period during which a result of the search for theat least one candidate cooperative device for the source device isvalid.
 3. The method of claim 1, wherein the valid time indicates a timeperiod during which a neighbor candidate cooperative device list of thesource device received from the source device is valid.
 4. The method ofclaim 1, wherein the valid time is the same or different from each otherfor each candidate cooperative device among the candidate cooperativedevice list.
 5. The method of claim 4, wherein, if the valid time is thesame for each of candidate cooperative devices among the candidatecooperative device list, the valid time is one of a minimum value, anaverage value, or a maximum value of valid time of each candidatecooperative device among the candidate cooperative device list.
 6. Themethod of claim 1, wherein whether the candidate cooperative device listis valid is determined when the list valid period timer expires.
 7. Themethod of claim 1, wherein whether the candidate cooperative device listis valid is determined when an estimation of a location of the sourcedevice exceeds a predetermined range, or moving speed of the sourcedevice is changed.
 8. The method of claim 1, wherein whether thecandidate cooperative device list is valid is determined when it isdetermined that the candidate cooperative device list is outdated basedon a timestamp value of the candidate cooperative device list.
 9. Themethod of claim 1, further comprising: starting a list holding timerwithin the base station; and holding information on the candidatecooperative device list and the source device for a time periodindicated by the list holding timer.
 10. The method of claim 9, whereintime indicated by the list holding timer is defined by a pre-fixedvalue.
 11. The method of claim 9, further comprising: transmitting thetime period indicated by the list holding timer to the source device.12. The method of claim 9, wherein the list holding timer starts whenthe candidate cooperative device list is updated.
 13. The method ofclaim 9, if the list holding timer expires, further comprising: deletingthe information on the candidate cooperative device list and the sourcedevice.
 14. The method of claim 1, wherein the first system is one ofinstitute of electrical and electronics engineers (IEEE) 802.16, IEEE802.16m, IEEE 802.20, evolved-UMTS terrestrial radio access (E-UTRA),3^(rd) generation partnership project (3GPP) long-term evolution (LTE),or 3GPP LTE-A (advanced), and wherein the second system is IEEE 802.11.15. A base station for client cooperation in a wireless communicationsystem, the base station comprising: a radio frequency (RF) unit fortransmitting or receiving a radio signal; and a processor connected tothe RF unit, and configured to: search for at least one candidatecooperative device for a source device; start a list valid period timerindicating valid time of a candidate cooperative device list for thesource device; determine whether the candidate cooperative device listis valid; and restart the list valid period timer if the at least onecandidate cooperative device for the source device is discovered whilethe list valid period timer is operating, wherein the base station andthe source device are connected to each other through a first system,and wherein the base station and the candidate cooperation device areconnected to each other through a second system.
 16. The method of claim1, wherein the list valid period timer starts when the candidatecooperative device list for the source device is obtained while the listvalid period timer is in deactivated state.